Organic electron transport material and organic electroluminescent element using same

ABSTRACT

R1 represents an atomic group which has one or more of either or both of aryl and heteroaryl groups and may have one or more phosphine oxide groups, R2 to R11 each independently represent an atom or an atomic group selected from the group consisting of a hydrogen atom, a halogen atom, a cyano group, a nitro group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, and a trifluoromethyl group.

TECHNICAL FIELD

The present disclosure relates to a novel organic electron transport material that is excellent in stability and durability and an organic electroluminescent element using the same.

BACKGROUND ART

Organic electroluminescence (EL) elements, in which a luminescent organic compound layer (organic electroluminescence layer) is provided between a positive electrode and a negative electrode, (hereinafter referred to as “organic EL elements”) are more advantageous than inorganic EL elements in that organic EL elements can be driven at low DC voltages and they have high brightness and luminescent efficiency. Therefore, the organic EL elements have been gaining attention as next-generation display devices. Recently, a full-color display panels are commercially available. Research and development have been actively made for an increase in the size of display screen, improvement of durability, and the like.

The organic EL element is an electroluminescence element in which an organic compound is electrically excited by recombination of the injected electrons and holes (positive holes) such that light emission is induced. Many companies and research institutes have been studying organic EL elements since Tang et al. from Kodak reported that an organic layered thin film element emits light with brightness (see Non Patent Literature 1). The representative configuration of the organic EL element according to Kodak was made by sequentially layering a diamine compound serving as a hole transporting material, tris (8-quinolinolato) aluminum (III) serving as a luminescent material, a cathode Mg:Al serving as a negative electrode on an ITO (indium tin oxide) glass substrate serving as a transparent positive electrode. Green light emission with a brightness of 1000 cd/cm² was observed at about 10V driving voltage. Basically, layered-type organic EL elements under current study or in practical use follow the configuration of the Kodak.

Organic EL elements are roughly divided by their constituent materials into polymer organic EL elements and the low-molecular organic EL elements. The former is produced by a wet method, and the latter is produced by a dry method or a wet method such as a vapor deposition method. For polymer organic EL elements, it is difficult to balance hole transport properties and electron transport properties of a conductive polymer material used for manufacturing of the elements. Therefore, in recent years, layered-type low-molecular organic EL elements in which the functions of electron transport, hole transport, and luminescence are separated are becoming the mainstream.

In the wet method, in which manufacturing is carried out using a printing technique such as an inkjet method, since it is possible to coat a large area at a time, even an element having a large screen can be easily produced with high productivity. Therefore, the method is expected to be used as a next-generation organic EL element manufacturing method that can replace the dry method. Methods of manufacturing layered-type low-molecular organic EL elements based on the wet method are roughly divided into two types. One is a method comprising conducting crosslinking or polymerization for insolubilization by heat or light after film formation of a lower layer, thereby conducting film formation of an upper layer. The other one is a method comprising using materials having significantly different solubilities for a lower layer and an upper layer. In the former method, although the selection of materials is wide, it is difficult to remove the reaction initiator and unreacted substances after crosslinking or polymerization, which is problematic in terms of durability. In the latter method, although the selection of materials is difficult, it is possible to construct a highly durable element with high purity because no chemical reaction is required as compared with the method involving insolubilization. In view of such a risk of chemical reaction, a suitable mode of layering by the wet method is considered to be layering based on the difference in solubility, although the selection of material is difficult. However, one of the factors that make it difficult to conduct layering utilizing the difference in solubility is the fact that most of conductive polymers and spin-coatable organic semiconductors are soluble only in solvents having relatively high levels of solvent power such as high toluene, chloroform, and tetrahydrofuran. In most cases, layered-type organic EL elements are manufactured by sequentially layering a hole transport layer comprising a P-type organic semiconductor, a light emitting layer, an electron transport layer comprising an N-type organic semiconductor on a positive electrode comprising a transparent metal oxide such as ITO. In this case, after the film formation of the hole transport layer with a P-type organic semiconductor polymer, an N-type organic semiconductor polymer is spin-coated using the same solvent. This causes the hole transport layer to be eroded, thereby making it impossible to form a flat layered structure having a PN interface with fewer defects. This is problematic. In particular, in the case of using an inkjet method, since the solvent is removed by natural drying, erosion of a hole transport layer and a light emitting layer is intensified violent, which might make it very difficult to obtain a device free of problems in practical use.

In order to solve the above-mentioned problems of organic EL production by a wet method, the inventors of the disclosure found that electron transport properties and electron injection properties can be further improved and durability can be remarkably improved by coordinating a metal having a small electronegativity (1.6 or less) to a phosphine oxide derivative having excellent electron transport properties and electron injection properties. Based on the findings, the inventors developed an electron transport material including an alcohol-soluble phosphine oxide derivative and a metal having an electronegativity of 1.6 or less (see Patent Literature 1). By using this material, it is possible to use alcohol for film formation of an electron transport layer by a wet method. This makes it possible to construct an electron transport layer without damaging the light-emitting layer comprising a luminescent polymer.

CITATION LIST Patent Literature

-   Patent Literature 1: Unexamined Japanese Patent Application Kokai     Publication No. 2010-278376

Non Patent Literature

-   Non Patent Literature 1: C. W. Tang, S. A. Van Slyke, “Organic     electroluminescent diodes”, Applied Physics Letters (US), The     American Institute of Physics, Sep. 21, 1987, Vol. 51, No. 12, pp.     913-915

SUMMARY OF INVENTION Technical Problem

However, an organic EL element, in which a phosphine oxide derivative which is not metal-doped with a metal is used as an electron transport material or a host material of a light emitting layer, has very short lifetime. This is one of the factors that make it difficult to apply a phosphine oxide derivative to the light emitting layer. Lin et al. reported based on the first-principles calculation results obtained by the density functional theory method that the chemical instability of a C—P bond in the anion state that received an injection of electrons is a factor of the instability of a phosphine oxide derivative used for the electron transport material or the host material for the light emitting layer (Na Lin, Juan Qiao, Lian Duan, Haifang Li, Liduo Wang, and Yong Qiu, J. Phys. Chem. C, 2012, 116(36), pp 19451-19457).

The present disclosure has been made in view of the above circumstances. An objective of the disclosure is to provide a novel organic electron transport material that is excellent in stability and durability, which is obtained by improving chemical stability of a C—P bond in the anion state, and an organic electroluminescent element using the same.

Solution to Problem

A first aspect of the disclosure is to provide an organic electron transport material, which comprises a phosphine oxide derivative represented by the following Formula (1), thereby achieving the above objective.

In Formula (1),

R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms,

Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and

at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms.

In Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.

The second aspect of the disclosure is to provide an organic electroluminescent element, which comprises: a pair of electrodes composed of a positive electrode and a negative electrode; and a light emitting layer that is formed between the pair of electrodes so as to allow holes and electrons to be injected from the electrodes directly or via a different thin film layer and is electrically excited by recombination of holes and electrons injected from the electrodes so as to emit light, wherein the organic electroluminescent element contains a phosphine oxide derivative represented by the following Formula (1) and includes an electron transport layer that is disposed to be in contact with the negative electrode side of the light emitting layer, thereby achieving the above objective.

In Formula (1),

R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms,

Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and

at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms.

In Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.

The third aspect of the disclosure is to provide an organic electroluminescent element, which comprises: a pair of electrodes composed of a positive electrode and a negative electrode; a compound that is formed between the pair of electrodes so as to allow holes and electrons to be injected from the electrodes directly or via a different thin film layer and is electrically excited by recombination of holes and electrons injected from the electrodes so as to emit light; and a light emitting layer that contains a phosphine oxide derivative represented by the following Formula (1), thereby achieving the above objective.

In Formula (1),

R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms,

Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and

at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms.

In Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.

In the first to third aspects of the disclosure, the electron withdrawing group may be selected from the group consisting of a halogen atom, a cyano group, a nitro group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, and a trifluoromethyl group.

Advantageous Effects of Invention

According to the disclosure, a novel organic electron transport material that is excellent in safety and durability because of high chemical stability of a C—P pond in the anion state and an organic electroluminescent element using the same are provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an organic EL element according to Embodiment 2 of the disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the disclosure are described with reference to the drawings.

Embodiment 1

An organic electron transport material according to Embodiment 1 of the disclosure (hereinafter simply referred to as “organic electron transport material” in some cases) is a phosphine oxide derivative represented by the following Formula (1).

In Formula (1),

R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms,

Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and

at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms.

In Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the above-mentioned electron withdrawing group.

The numbers of carbons for an aryl group and a heteroaryl group contained in R¹ are preferably, but are not particularly limited to, from 2 to 30. Specific examples include: monocyclic aromatic hydrocarbon groups such as a phenyl group; monocyclic heterocyclic groups such as a thiophene ring, a triazine ring, a furan ring, a pyrazine ring, a pyridine ring, a thiazole ring, an imidazole ring, and a pyrimidine ring; condensed polycyclic aromatic hydrocarbon groups such as a naphthalene ring, an anthracene ring, a fluorene ring, and a spirobifluorene ring; condensed polycyclic heterocyclic ring groups such as thieno[3,2-b]furan ring; ring-assembled aromatic hydrocarbon groups such as a biphenyl ring and a terphenyl ring; ring-assembled heterocyclic groups such as a bithiophene ring and a bifuran ring; and a combinations of a heterocyclic ring and an aromatic ring such as an acridine ring, an isoquinoline ring, an indole ring, a carbazole ring, a carboline ring, a quinoline ring, a dibenzofuran ring, a cinnoline ring, a thionaphthene ring, a 1,10-phenanthroline ring, a phenothiazine ring, a purine ring, a benzofuran ring, or a silole ring. Examples of an aryl group and a heteroaryl group contained in Ar¹ and Ar² may be an aryl group and a heteroaryl group contained in the above-mentioned R¹. However, a phenyl group or a phenanthyl group (for example, a 9-phenanthyl group) are preferable. In Ar¹ and Ar², an atom or group substituted by an aryl group and/or a heteroaryl group is preferably an electron withdrawing group, which will be described later.

The electron withdrawing group is a group having a positive Hammett σ value (log [K_(A)/K_(A0)]: K_(A) is pKa of substituted benzoic acid, K_(A0) is pKa of benzoic acid). Specific examples may include: a halogen atom such as a fluorine atom which is located at the m-position relative to a phosphine oxide group; and a haloalkyl group such as a nitro group, a cyano group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, or a trifluoromethyl group which is located at the m-position or p-position relative to a phosphine oxide group. Of these, a halogen atom and a cyano group are preferable, and the halogen atom is preferably a fluorine atom.

A specific example of preferable electron transport material is one represented by any of the following Formulae (I) to (XI).

Examples of other preferable compounds include those mentioned below.

For synthesis of an organic electron transport material, a publicly known method can be employed. The organic electron transport material can be obtained by condensing a compound (3) represented by Formula (3) (diaryl phosphine oxide derivative) and a compound (4) represented by the following Formula (4) in a solvent in the presence of a condensation catalyst and/or a base (dehydrohalogenation reaction).

In the above formula, a case in which the compound (4) has a single leaving group X per molecule is shown. However, the compound may have a plurality of leaving groups X per molecule. Specific examples of leaving groups X that can be used herein include a chlorine atom, a bromine atom, an iodine atom, a triflyl group (trifluoromethanesulfoxyl group), and a tosyl group (toluenesulfoxyl group).

To obtain a 1-substituted compound represented by Formula (1) above, the amount of the compound (3) to be used relative to 1 mol of the compound (4) is about from 1.0 to 4 mol.

For a compound in which two or more molecules of phosphine oxide were substituted, the amount of compound (3) to be used may be multiplied by n in accordance with the substitution number n.

Examples of the solvent include C₁-C₈ alcohols, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), tetrahydrofuran (THF), dioxane, pyridine, benzene, toluene, and xylene. Of these, DMSO is preferred from the viewpoint of yield.

Examples of the condensation catalyst include, for example, complex compounds of bisphosphinoalkane with palladium acetate [Pd(OAc)₂], nickel acetate [Ni(OAc)₂], and platinum group elements such as Pd(OAc)₂-1,3-bis(diphenylphosphino)propane[dppp], Pd(OAc)₂-1,2-bis(diphenylphosphino)ethane[dppe], Pd(OAc)₂-1,4-bis(diphenylphosphino)butane[dppb], Ni(OAc)rdppe, and Ni(OAc)₂-dppp. Of these, Pd(OAc)₂, Pd(OAc)-dppp, and Pd(OAc)₂-dppb are preferable from the viewpoint of yield.

In addition to the above, tetrakis(triphenylphosphine) palladium (0) and [1,1′-bis(diphenylphosphino)ferrocene]palladium (II), which are commonly used in cross-coupling can also be used. The amount of the catalyst to be used is about from 0.005 to 0.1 mol relative to 1 mole of the compound (4).

The above base has a function of capturing the hydrogen halide generated by condensation. Examples of such a base include: trialkylamines such as triethylamine, tripropylamine, and tributylamine; aliphatic tertiary amines such as N-ethyldiisopropylamine [DIEA]; and aromatic tertiary amines such as pyridine and N, N′-dimethylaminopyridine [DMAP], of these, DIEA and DMAP are preferable from the viewpoint of boiling point. The amount of the base to be used is an amount sufficient to capture the theoretical amount of hydrogen halide generated, which is about from 1.0 to 2 mol relative to 1 mol of the compound (4).

The reaction can be carried out at a reaction temperature of 80° C. to 180° C. From the viewpoint of yield, the reaction temperature is preferably from 100° C. to 160° C.

More specifically, the organic electron transport material may be synthesized by, for example, by Schemes 1 to 11 described below.

A C—N bond between a benzene ring and a nitrogen atom of carbazole can be formed by a coupling reaction of a fluorobenzene derivative and carbazole in the presence of sodium hydride in DMF (see Scheme 1). A C—P bond between a carbon atom of 2,4,6-triphenyl-1,3,5-triazine, 2, 6-diphenyl-4-(4-phenylphenyl)pyridine, 9,9′-spirofluorene and a fluorine atom or a phosphorus atom of a diphenylphosphine oxide derivative having a cyano group can be formed by a coupling reaction of 2,4,6-triphenyl-1,3,5-triazine, 2, 6-diphenyl-4-(4-phenylphenyl)pyridine, 5-(carbazole-9-yl)-benzene, 9,9′-spirofluorene substituted by a bromine atom in the presence of palladium acetate, dppp, diisopropylethylamine, and DMSO and the diphenylphosphine oxide derivative described above (see Schemes 2 to 6). A C—P bond between a carbon atom of a fluorobenzene derivative, a benzonitrile derivative, or an isophthalonitrile derivative and a phosphorus atom of 1,4-bis(phenylphosphinoyl)benzene, 2,7-bis(phenylphosphinoyl)-9,9-dimethylfluorene, diphenanthrylphosphine oxide can also be formed in the same manner by a coupling reaction of fluorobenzene, benzonitrile, or isophthalonitrile substituted by a bromine atom in the presence of palladium acetate, dppp, diisopropylethylamine, and DMSO and 1,4-bis(phenylphosphinoyl)benzene, 2,7-bis(phenylphosphinoyl)-9,9-dimethylfluorene, diphenanthrylphosphine oxide (see Schemes 7 to 11).

In an organic electroluminescent element or the like represented by the organic EL element 1, an electron transport material can be used as a host material (electron transport material) of the light emitting layer 5 or a constituent material of the electron transport layer 6 independently or in combination with other one or a plurality of compounds.

Embodiment 2: Organic Electroluminescent Element

An organic EL element 1, which is one example of the organic electroluminescent element according to Embodiment 2 of the disclosure include a plurality of organic compound layers (in the order of a hole transport layer 4, a light emitting layer 5, and an electron transport layer 6 from the side of a positive electrode 3 in the organic EL element 1 according to the present embodiment), which is layered such that it is sandwiched between the positive electrode 3 and a negative electrode 7 formed on a transparent substrate 2, and the organic EL element as a whole is sealed with the sealing member 8.

Note that the expression “Y (an electrode or organic compound layer) is provided on X (an electrode or organic compound layer)” means that “Y is present on the surface on the side of negative electrode 7 of X such that the surface of X and the surface of Y are in contact with each other,” and the expression “Y (an electrode or organic compound layer) is formed on X (an electrode or organic compound layer)” means that “Y is formed on the surface on the side of negative electrode 7 of X such that the surface of X and the surface of Y are in contact with each other.”

First, the transparent substrate 2, positive electrode 3, and negative electrode 7 that constitute the organic EL element 1 will be explained.

[Transparent Substrate 2]

The transparent substrate 2 serves as a support of the organic EL element 1. The organic EL element 1 according to the present embodiment has a configuration (bottom emission type) in which light is extracted from the side of the transparent substrate 2. Therefore, the transparent substrate 2 and the positive electrode 3 each are formed with a substantially transparent (colorless transparent, colored transparent, translucent) material. Examples of the constituent material of the transparent substrate 2 include, for example, resin materials such as polyethylene terephthalate, polyethylene naphthalate, polypropylene, a cycloolefin polymer, polyamide, polyether sulfone, polymethyl methacrylate, polycarbonate, and polyarylate, and glass materials such as quartz glass and soda glass, which can be used singly or in combination of two or more thereof.

The average thickness of the transparent substrate 2 is, but is not particularly limited to, preferably from about 0.1 to 30 mm and more preferably from about 0.1 to 10 mm. In a case in which the organic EL element 1 has a configuration (top emission type) in which light is extracted from the side opposite the transparent substrate 2, an opaque substrate is sometimes used instead of the transparent substrate 2. Examples of the opaque substrate include a substrate formed with a ceramic material such as alumina, a surface obtained by forming an oxide film (insulating film) on a metal substrate such as stainless steel, and a substrate made of a resin material.

[Positive Electrode 3]

The positive electrode 3 is an electrode for injecting holes into the hole transport layer 4 described later. It is preferable to use, as a constituent material of positive electrode 3, a material that has a large work function and is excellent in electrical conductivity. Examples of the constituent material of the positive electrode 3 include, for example, oxides such as ITO (indium tin oxide), IZO (indium zirconium oxide), In₃O₃, SnO₂, Sb-containing SnO₂, and Al-containing ZnO, Au, Pt, Ag, Cu, or alloys containing these examples, which can be used singly or in combination of two or more thereof. The average thickness of the positive electrode 3 is, but is not particularly limited to, preferably from about 10 to 200 nm and more preferably from about 50 to 150 nm.

[Negative Electrode 7]

Meanwhile, the negative electrode 7 is an electrode for injecting electrons into the electron transport layer 6 described later, which is provided on the side opposite the light emitting layer 5 of the electron transport layer 6. As the constituent material of the negative electrode 7, it is preferable to use a material having a low work function. Examples of the constituent material of the negative electrode 7 include, for example, Li, Mg, Ca, Sr, La, Ce, Er, Eu, Sc, Y, Yb, Ag, Cu, Al, Cs, Rb, or alloys comprising these examples, which can be used in combination of arbitrary one or two or more thereof (for example, a layered product including a plurality of layers).

In particular, in a case in which an alloy is used as a constituent material of the negative electrode 7, it is preferable to use an alloy containing a stable metal element such as Ag, Al, or Cu, which is specifically an alloy such as MgAg, AlLi, or CuLi. With the use of such an alloy as a constituent material of the negative electrode 7, it is possible to improve the electron injection efficiency and stability of the negative electrode 7. The average thickness of the negative electrode 7 is, but is not particularly limited to, preferably from about 50 to 10000 nm and more preferably from about 80 to 500 nm.

In the case of the top emission type, the thickness of a material having a low work function or an alloy containing such a material is set to from about 5 to 20 nm, thereby allowing the material to have a permeability, and further, a highly permeable conductive material such as ITO is formed on the upper surface of the material to result in a thickness of from about 100 to 500 nm.

Note that since the organic EL element 1 according to the present embodiment is of the bottom emission type, the negative electrode 7 is not particularly required to have light transmissivity.

Next, organic compound layers (in the order of the hole transport layer 4, the light emitting layer 5, and the electron transport layer 6 from the side of the positive electrode 3) that constitutes the organic EL element 1 is described.

The hole transport layer 4 is provided on the positive electrode 3. The hole transport layer 4 functions to transport holes injected from the positive electrode 3 to the light emitting layer 5.

[Hole Transport Layer 4]

Specific examples of a constituent material of the hole transport layer 4 include: metal or metal-free phthalocyanine compounds such as phthalocyanine, copper phthalocyanine (CuPc), and iron phthalocyanine; polyarylamine such as polyaniline; aromatic amine derivatives; fluorene-arylamine copolymers, fluorene-bithiophene copolymers, poly(N-vinylcarbazole), polyvinyl pyrene, polyvinyl anthracene, polythiophene, polyalkylthiophene, polyhexylthiophene, poly(p-phenylenevinylene), polyethylenevinylene, pyrene formaldehyde resins, ethylcarbazole formaldehyde resins, or derivatives thereof. These examples can be used in combination of one or two or more thereof in the form of a mixture or layered product.

Specific examples of aromatic amine derivatives include the following compounds.

In addition, the above compounds may also be used in the form of a mixture with other compounds. One example of a mixture containing polythiophene is poly(3,4-ethylenedioxythiophene/styrenesulfonic acid) (PEDOT/PSS) or the like.

The average thickness of the hole transport layer 4 is, but is not particularly limited to, preferably from about 10 to 150 nm and more preferably from about 50 to 100 nm.

[Light Emitting Layer 5]

The light emitting layer 5 is provided on the hole transport layer 4, which means the surface on the side of the negative electrode 7. Into this light emitting layer 5, electrons from the electron transport layer 6 described later and holes from the hole transport layer 4 are supplied (injected). Then, holes and electrons are recombined, the energy released upon the recombination generates excitons, and the energy is released (emitted) when the excitons return to the ground state (fluorescence or phosphorescence) within the light emitting layer 5.

Among constituent materials of the light emitting layer 5, specific examples of the luminescent substance (guest material) having a light-emitting function include: benzene compounds such as 1,3,5-tris[(3-phenyl-6-trifluoromethyl)quinoxalin-2-yl] benzene (TPQ1) and 1,3,5-tris[{3-(4-t-butylphenyl)-6-trifluoromethyl}quinoxalin-2-yl]benzene (TPQ2); low-molecular compounds such as tris(8-hydroxyquinolate)aluminum (Alq₃) and factris(2-phenylpyridine)iridium (Ir(ppy)₃); and polymer compounds such as an oxadiazole polymer, a triazole polymer, a carbazole polymer, a polyfluorene polymer, and a polyparaphenylene vinylene polymer, which can be used in combination of two or more thereof.

In addition, the light emitting layer 5 contains, as a material responsible for the charge transport of electrons or holes (host material), an organic electron transport material according to Embodiment 1 of the present disclosure. A guest material is uniformly distributed in a host material. The concentration of the guest material is generally about 0.1 to 1% by weight of the host material.

The light emitting layer 5 may further contain an electron transport auxiliary material and a hole transport auxiliary material, in addition to the above-mentioned host material and guest material.

Specific examples of an electron transport auxiliary material and a hole transport auxiliary material that can be added to the light emitting layer 5 include arbitrary materials used as constituent materials of the hole transport layer 4 and the electron transport layer 6 described later and any combination of two or more thereof.

The average thickness of the light emitting layer 5 is, but is not particularly limited to, preferably from about 10 to 150 nm and more preferably from about 50 to 100 nm.

[Electron Transport Layer 6]

The electron transport layer 6 is provided on the light emitting layer 5. The electron transport layer 6 functions to transport holes injected from the negative electrode 7 to the light emitting layer 5.

Examples of constituent materials of the electron transport layer 6 include: various metal complexes such as metal complexes of triazole derivatives, oxazole derivatives, polycyclic compounds, hetero polycyclic compounds such as bathocuproine, oxadiazole derivatives, fluorenone derivatives, diphenylquinone derivatives, thiopyran dioxide derivatives, anthraquinone dimethane derivatives, anthrone derivatives, carbodiimide derivatives, fluorenylidene methane derivatives, distyryl pyrazine derivatives, acid anhydrides of aromatic tetracarboxylic acids such as naphthalene tetracarboxylic acid or perylenetetracarboxylic acid, phthalocyanine derivatives, and 8-quinolinol derivatives; and metal complexes in which metal phthalocyanine, benzoxazole, or benzothiazole is a ligand, organic silane derivatives, iridium complexes, phosphine oxide derivatives such as alcohol-soluble phosphine oxide derivative disclosed in Unexamined Japanese Patent Application Kokai Publication No. 2010-278376, and any combination of two or more of these compounds. Alternatively, an electron transport material according to Embodiment 1 of the present disclosure may be used singly or in combination of another compound described above as a constituent material of the electron transport layer 6.

The average thickness of the electron transport layer 6 is, but is not particularly limited to, preferably from about 1 to 100 nm and more preferably from about 10 to 50 nm.

The sealing member 8 is provided so as to cover the organic EL element 1 (the positive electrode 3, hole transport layer 4, light emitting layer 5, electron transport layer 6, and negative electrode 7), thereby airtightly sealing it and having a function of blocking oxygen and moisture. By providing the sealing member 8, it is possible to obtain the effects of improving reliability of the organic EL element 1 and preventing alteration and deterioration (improving durability), for example.

Examples of constituent materials of the sealing member 8 include, for example, Al, Au, Cr, Nb, Ta, or Ti and alloys containing these examples, silicon oxide, and various resin materials. In the case of using a material having conductivity as a constituent material of the sealing member 8, in order to prevent a short circuit, an insulating film preferably provided between the sealing member 8 and the organic EL element 1 if necessary. In addition, the sealing member 8 may be in the form of a flat plate such that it faces the transparent substrate 2 and a space therebetween is sealed with a sealant such as a thermosetting resin.

For example, an organic EL element 1 can be produced in the following manner.

First, a transparent substrate 2 is prepared, and a positive electrode 3 is formed on this transparent substrate 2. The positive electrode 3 can be formed by, for example, using a chemical vapor deposition (CVD) method such as plasma CVD, thermal CVD, or laser CVD, a dry plating method such as vacuum vapor deposition, sputtering, or ion plating, a wet plating method such as electrolytic plating, immersion plating, or electroless plating, thermal spraying method, a sol-gel method, an MOD method, or bonding of metallic foil.

Next, a hole transport layer 4 is formed on the positive electrode 3. The hole transport layer 4 can be deposited using a vacuum vapor deposition method or a coating method. By using a coating method, for example, the hole transport layer 4 can be formed by supplying a hole transport material dissolved in a solvent or dispersed in a dispersion medium on the positive electrode 3, followed by drying (removal of the solvent or dispersion medium). As a method of supplying a material for forming a hole transport layer, for example, various coating method such as a spin coating method, a casting method, a micro-gravure coating method, a gravure coating method, a bar coating method, a roll coating method, a wire bar coating method, a dip coating method, a spray coating method, a screen printing method, a flexo printing method, an offset printing method, or an inkjet printing method can be used. By using such a coating method, it is possible to relatively easily form the hole transport layer 4.

The solvent or dispersion medium used for dissolution or dispersion is appropriately selected depending on the solubility of the hole transport material to be used, cost, availability, the ease of drying, safety, and the like. Specific examples of the solvent or dispersion medium are the following various organic solvents or mixed solvents containing them and the like, which include: inorganic solvents such as nitric acid, sulfuric acid, ammonia, hydrogen peroxide, water, carbon disulfide, carbon tetrachloride, and ethylene carbonate; ketone solvents such as methyl ethyl ketone (MEK), acetone, diethyl ketone, methyl isobutyl ketone (MIBK), methyl isopropyl ketone (MIPK), and cyclohexanone; alcohol solvents such as methanol, ethanol, isopropanol, ethylene glycol, diethylene glycol (DEG), and glycerin; ether solvents such as diethyl ether, diisopropyl ether, 1,2-dimethoxyethane (DME), 1,4-dioxane, tetrahydrofuran (THF), tetrahydropyran (THP), anisole, diethyleneglycol dimethyl ether (diglyme), and diethyleneglycol ethyl ether (carbitol); cellosolve solvents such as methyl cellosolve, ethyl cellosolve, and phenyl cellosolve; aliphatic hydrocarbon solvents such as hexane, pentane, heptane, and cyclohexane; aromatic hydrocarbon solvents such as toluene, xylene, and benzene; aromatic heterocyclic compound solvents such as pyridine, pyrazine, furan, pyrrole, thiophene, and methyl pyrrolidone; amide solvents such as N,N-dimethyl formamide (DMF), N,N-dimethylacetamide (DMA); halogen compound solvents such as chlorobenzene, dichloromethane, chloroform, and 1,2-dichloroethane; ester solvents such as ethyl acetate, methyl acetate, and ethyl formate; sulfur compound solvents such as dimethyl sulfoxide (DMSO) and sulfolane; nitrile solvents such as acetonitrile, propionitrile, acrylonitrile; and organic acid solvents such as formic acid, acetic acid, trichloroacetic acid, and trifluoroacetic acid. Drying may be carried out by, for example, standing in an atmospheric pressure or reduced pressure atmosphere, heat treatment, spraying of inert gas, or the like.

Further, oxygen plasma treatment may be performed on the upper surface of the positive electrode 3 prior to this step. This allows imparting lyophilic properties to the upper surface of the positive electrode 3, removing organic matter attached to the upper surface of the positive electrode 3 (washing), adjusting the work function in the vicinity of the upper surface of the positive electrode 3, and the like. Here, preferable conditions for oxygen plasma treatment include, for example, the following: plasma power: about 100 to 800 W; oxygen gas flow rate: about 50 to 100 mL/min; processed member (positive electrode 3) transport rate: about 0.5 to 10 mm/second; and temperature of transparent substrate 2: about 70° C. to 90° C.

Next, a light emitting layer 5 is formed on the hole transport layer 4 (on one surface side of the positive electrode 3).

A light emitting layer 5 can be deposited by a vacuum vapor deposition method or a coating method. By using a coating method, for example, the light emitting layer 5 can be formed by supplying a host material dissolved in a solvent or dispersed in a dispersion medium on the hole transport layer 4, followed by drying (removal of the solvent or dispersion medium). It is desirable to select as a solvent or dispersion medium to be used, a solvent or dispersion medium that does not allow the hole transport layer 4 to be dissolved, eroded, or swollen.

A method of drying a host material and a guest material dispersed in a solvent or dispersion medium is the same as that described in the formation of the hole transport layer 4.

Next, an electron transport layer 6 is formed on the light emitting layer 5 (on one surface side of positive electrode 3).

The electron transport layer 6 can be deposited using a vacuum vapor deposition method or a coating method as in the case of the hole transport layer 4 and the light emitting layer 5. By using a coating method, the electron transport layer 6 can be obtained by supplying an electron transport material dissolved in a solvent or dispersed in a dispersion medium on the light emitting layer 5, followed by drying. It is desirable to select as a solvent or dispersion medium to be used, a solvent or dispersion medium that does not allow the light emitting layer 5 to be dissolved, eroded, or swollen. A method of drying an electron transport material dissolved in a solvent or dispersion medium is the same as that described in the formation of the hole transport layer 4 and the light emitting layer 5. Therefore, a detailed description thereof is omitted.

Next, a negative electrode 7 is formed on the electron transport layer 6 (on the side opposite the light emitting layer 5).

The negative electrode 7 can be deposited using, for example, a vacuum vapor deposition method, a sputtering method, bonding of metallic foil, coating with metal fine particle ink, and baking.

Eventually, a sealing member 8 is placed to cover the obtained organic EL element 1 and bonded to the transparent substrate 2.

Through the above steps, the organic EL element 1 is obtained.

According to the above manufacturing method, upon the formation of an organic layer (hole transport layer 4, light emitting layer 5, or electron transport layer 6) or the formation of the negative electrode 7 in the case of using metal fine particle ink, large-scale equipment such as a vacuum device is not required. Therefore, it is possible to reduce the time and cost for manufacturing the organic luminescent element 1. Further, the application the inkjet method (droplet discharge method) makes it easy to produce a large-area element or separately apply many colors.

In the present embodiment, the case of manufacturing the hole transport layer 4 and the light emitting layer 5 by using a liquid phase method is described. However, these layers may be formed by, for example, a vapor phase process such as a vacuum vapor deposition method depending on types of a hole transport material and a luminescent material to be used.

Such an organic EL element 1 can be used as, for example, a light source or the like. Further, by arranging a plurality of organic EL elements 1 in a matrix, it is possible to construct a display device. As a method of driving a display device is not particularly limited, and it may be either an active matrix method or a passive matrix method.

A source of electrical energy supplied to the organic EL element 1 is mainly a direct current while it is also possible to use a pulse current or alternating current. Although the current and voltage values are not particularly limited, it is desirable to achieve maximum brightness at the lowest possible energy in view of the power consumption and lifetime of the element.

A “matrix” that constitutes a display device is a device in which pixels are disposed in a grid pattern for the display purpose, and characters and images are displayed by sets of pixels. The shape and size of a pixel are determined by its application. For example, square pixels having a side of 300 μm or less are usually used for the image and character display of personal computers, monitors, and TV. For large displays such as display panels, pixels having a millimeter-order side length are used. For monochrome display, pixels of the same color may be arranged. However, in the case of color display, red, green, and blue pixels are arranged for display. In this case, typically, there are delta and stripe arrangements. A method of driving the matrix may be either a passive matrix method or an active matrix method. Although the former passive matrix is advantageous in that the structure is simple, the latter active matrix is superior thereto in consideration of operating characteristics in some cases. It is therefore necessary to select the method depending on the usage.

The organic EL element 1 may be a segment-type display device. The term “segment type” means that a pattern having a predetermined shape is formed to allow a specific region to emit light so as to display a given information. For example, segment-type display includes time and temperature display in a digital clock or thermometer, operating state display in an audio equipment, an electromagnetic cooker, or the like, and automobile panel display. The matrix display and the segment display may coexist in the same panel.

The organic EL element 1 may be a backlight that is used for liquid crystal display devices, watches, audio equipment, automobile panels, display panels, signs, and the like in order to improve the visibility of a non-self luminous display device. In particular, when the organic EL element 1 is used as a backlight for a liquid crystal display device and especially a personal computer for which there is an expectation to realize a thinner display device, the organic EL element 1 will achieve a thinner and lighter display device as compared with the conventional one composed of a fluorescent lamp and a light guide plate.

Such an organic EL element 1 can be used as, for example, a light source or the like. Further, by disposing a plurality of organic EL elements 1 by arranging in a matrix, it is possible to construct a display device.

As a method of driving a display device is not particularly limited, and it may be either an active matrix method or a passive matrix method.

In the organic EL element 1, it is possible to provide an electron injection layer (not shown) between the negative electrode 7 and the electron transport layer 6. The electron injection layer is used for the improvement of efficiency of electron injection from the negative electrode 7 to the electron transport layer 6, which means reduction of driving voltage. It is preferable to use at least one metal compound selected from the group consisting of an alkali metal chalcogenide, an alkaline earth metal chalcogenide, an alkali metal halide, an alkaline earth metal halide, an alkali metal carbonate, an alkaline earth metal carbonate, an alkali metal hydrogen carbonate, an alkaline earth metal, and hydrogen carbonate for the electron injection layer. When the electron injecting layer is formed with such as alkali metal chalcogenide and the like, it is preferable in that it is possible to further improve the electron injection property. Specific examples of a preferred alkali metal chalcogenide include, for example, lithium oxide (Li₂O), potassium oxide (K₂O), sodium sulfide (Na₂S), sodium selenide (Na₂Se), and sodium oxide (Na₂O). Examples of a preferred alkaline earth metal chalcogenide include, for example, calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), beryllium oxide (BeO), barium sulfide (BaS), and calcium selenide (CaSe). Further, examples of a preferred alkali metal halide include, for example, lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), lithium chloride (LiCl), potassium chloride (KCl), and sodium chloride (NaCl). Furthermore, examples of a preferred alkaline earth metal halide include, for example, calcium fluoride (CaF₂), barium fluoride (BaF₂), strontium fluoride (SrF₂), magnesium fluoride (MgF₂), and beryllium fluoride (BeF₂), and also halides other than fluorides.

In addition to the above inorganic compounds, an organic metal complex containing an alkali metal and an alkaline earth metal can be used. Specific examples of a preferred organic metal complex include complexes of alkali metals and alkaline earth metals with ligands that include β-diketones such as acetylacetone, dibenzoylmethane, and 2,2,6,6-tetramethyl-3,5-heptanedione and heterocyclic rings such as 8-hydroxyquinoline and 2-picolinic acid.

In addition, a hole blocking layer (not shown) may be provided between the light emitting layer 5 and the electron transport layer 6. By providing the hole blocking layer, it is possible to suppress influx of holes into the electron transport layer 6 and also to increase the luminous efficiency and extend the lifetime of the organic EL element 1. The hole blocking layer used herein may be provided using the electron transport material described above. It is preferable to form the hole blocking layer as a mixed layer obtained by mixing and layering two or more kinds of electron transport materials by co-evaporation or the like. It is preferable to increase the ionization potential of the electron transport material contained in the hole blocking layer to a level greater than that of the light emitting layer 5.

It is also possible to provide a hole injection layer (not shown) between the hole transport layer 4 and the positive electrode 3. As a constituent material of the hole injection layer, any of those appropriately selected from the specific examples of the constituent material of the hole transport layer 4 described above can be used. However, a material that transports holes to the light emitting layer 5 at a lower field strength than that of the material of the hole transport layer 4 is preferable.

EXAMPLES

Next, the examples carried out for confirming the effects of the disclosure will be described below.

Example 1: Evaluation of Stability of Phosphine Oxide Derivatives in the Anion State by the First-Principles Calculation Based on the Density Functional Theory Method

Optimization of the structure and calculation of heat of formation were conducted on a workstation under the following conditions using GAMESS as the calculation program.

-   -   Neutral molecule, cation, anion fragment: RHF (restricted         Hartree-Fock method)/6-31G (d)/B3LYP     -   Cation, anion molecule, neutral fragment: UHF (unrestricted         Hartree-Fock method)/6-31G (d)/B3LYP

Single point calculation was performed on fragment molecules among the above based on the parent ion conformation.

Based on the results of heat of formation, bond dissociation energy (BDE) of a dissociation reaction represented by the following Formula (1) was calculated.

M→F1+F2  (1)

In this case, bond dissociation energy is obtained by the following Formula (2).

BDE=H _(M)−(H _(F1) +H _(F2))  (2)

BDE: Bond dissociation energy

H_(M): Heat of formation of the parent molecule

H_(F1), H_(F2): Heat of formation of the respective fragment molecules

It was confirmed that the calculation results of Lin et al. can be reproduced in a model compound system. Bond dissociation energy was calculated for the following compounds. Table 1 lists the calculation results of bond dissociation energy of a P—C bond between the phosphorus atom and the outer-shell phenyl group/difluorophenyl group for C_(z)PO2 which is the compound exemplified by Lin et al. and F4C_(z)PO2 into which fluorine was introduced as an electron withdrawing group. It was found that the introduction of the fluorine atom as an electron withdrawing group into the phosphine oxide derivative residue results in an increase of about 10 kcal/mol in BDE of the anion.

TABLE 1 Comparison of BDE between CzPO2 and fluorine-substituted F4CzPO2 BDE of neutral BDE of cation BDE of anion kcal/mol kcal/mol kcal/mol CzPO2 89.05 93.59 52.33 F4CzPO2 86.69 94.66 61.19

Example 2: Synthesis of the Basic Structure of Electron Transport Material Synthesis of 2,6-(p-bromophenyl)-4-biphenylpyridine

4-Phenyl benzaldehyde (4.61 g, 25.3 mmol), 4′-bromoacetophenone (10.1 g, 50.6 mmol), and ammonium acetate (25.1 g, 325 mmol) were added to acetic acid (63.4 mL) and refluxed for 9 hours. After the completion of the reaction, the mixture was cooled to room temperature, and precipitated crystals were collected by filtration and washed with ethanol. The crystals were recrystallized from dichloromethane/IPA to give 4.42 g of the title compound (yield: 32%). APCI-TOF-MS (m/z)=538, 540, 542

Synthesis of 2,4-bis(4-bromophenyl)-6-phenyl-1,3,5-triazine

Benzoyl chloride (3.51 g, 25 mmol) and 4-bromobenzonitrile (9.10 g, 50 mmol) were dissolved in chlorobenzene (37.5 mL) and cooled to 0° C. Antimony chloride (V) (7.48 g, 25 mmol) was added dropwise to the resulting solution and stirred at room temperature for 20 minutes and at 100° C. for 2 hours. After the completion of the reaction, the mixture was cooled to −20° C. and quenched with the addition of 25% aqueous ammonia (20 mL) during vigorous stirring. Chlorobenzene (25 mL) was added at room temperature, and water was removed by azeotropic distillation. The residue was hot-filtered and heated to 130° C. The residue on the filter paper was added to chloroform (25 mL) and hot-filtered again by heating. Methanol (50 mL) was added to the obtained filtrate, and the resulting precipitate was collected to give 7.00 g of the title compound (yield: 60%). APCI-TOF-MS (m/z)=465, 467, 469

Synthesis of 1,3-dibromo-5-(carbazole-9-yl)-benzene

55% Sodium hydride (500 mg, 1.5 mmol) was suspended in DMF (20 mL), carbazole (1.67 g, 10 mmol) was added, and the suspension was dissolved by heating to 80° C. Subsequently, 1,3-dibromo-5-fluorobenzene (2.54 g, 10 mmol) was added dropwise and stirred at 100° C. for 16 hours during heating. The reaction mixture was cooled to room temperature and quenched with ice water. The aqueous solution was extracted with dichloromethane. The resultant organic layer was concentrated under reduced pressure and dried over magnesium sulfate. The resulting residue was purified by column chromatography and recrystallized from heptane to give 1.63 g of the title compound (yield: 41%). APCI-TOF-MS (m/z)=402

Synthesis of 2,6-bis(4-(bis(3,5-difluorophenyl)phosphinoyl)phenyl)-4-(4-phenylphenyl)pyridine

2,4-Bis(4-bromophenyl)-4-phenyl-1,3,5-triazine (572 mg, 1.0 mmol), di(4-fluorophenyl)phosphine oxide (F2DPPO) (572 mg, 2.4 mmol), palladium acetate (22.5 mg, 0.10 mmol), dppp (82.5 mg, 0.20 mmol), and diisopropylethylamine (3.2 mL) were added to DMSO (6.4 mL) and stirred at 100° C. for 20 hours. After the completion of the reaction, the mixture was poured into a saturated ammonium chloride aqueous solution and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography to give 634 mg of a pale green solid. The resulting solid was recrystallized from cyclohexane/ethanol to give 348 mg of the title compound (yield: 44%). FAB-MS (m/z)=782

Synthesis of bis(3,5-difluorophenyl)phosphine oxide (F4DPPO)

To Mg (3.16 g, 130 mmol), 65 mL of a 1-bromo-3,5-difluorobenzene (25.0 g, 130 mmol)-ether solution was added dropwise over 30 minutes. After the completion of the dropwise addition, the mixture was stirred at room temperature for 2 hours. Next, cooling to −20° C. was performed, dichloro(dimethylamino)phosphine (9.49 g, 65 mmol) was added, and the mixture was stirred at −20° C. for 15 minutes and at room temperature for 1 hour. Subsequently, cooling in an ice bath was performed, 12N hydrochloric acid (24 mL, 780 mmol) was added, the mixture was stirred at 0° C. for 20 minutes and at room temperature for 17 hours. After the completion of the reaction, the mixture was diluted with dichloromethane, washed with water and a saturated NaHCO₃ aqueous solution, dried over magnesium sulfate, and concentrated under reduced pressure. The resulting residue was purified by column chromatography to give 10.0 g of the title compound (yield: 38%). APCI-TOF-MS (m/z)=275

Synthesis of di(4-cyanophenyl)phosphine oxide

A 1M diisopropyl magnesium chloride-lithium chloride complex-THF solution (34 mL, 34 mmol) was cooled to 0° C., 4-bromo-benzonitrile (6.19 g, 34 mmol) was added, and the mixture was and stirred at 0° C. Four hours later, a 1.5M dichloro dimethylamide phosphine-THF solution (13 mL, 19.5 mmol) was added dropwise, and the mixture was stirred for 2 hours. Subsequently, 12N HCl (8 mL) was added and stirred overnight at room temperature. After the completion of the reaction, the mixture was extracted with dichloromethane. The organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The resulting residue was purified by column chromatography. The resulting solid was recrystallized from cyclohexane to give 1.10 g of the title compound (yield: 26%).

Synthesis of di-9-phenanthrylphosphine oxide (DPhenPO)

A 9-bromophenanthrene (25.7 g, 100 mmol)-THF solution (50 mL) was added dropwise to Mg (2.43 g, 100 mmol) over 30 minutes. After the completion of the dropwise addition, the mixture was refluxed. Two hours later, cooling to room temperature was performed, THF (30 mL) was added, and the mixture was added dropwise to a phosphorus trichloride (7.55 g, 50 mmol)-THF solution (30 mL) cooled to 0° C. over 30 minutes. After the completion of dropwise addition, the mixture was stirred at room temperature for 18 hours. Subsequently, 1N hydrochloric acid (100 mL) was added and stirred at room temperature for 1 hour. The resulting precipitate was collected by filtration. The precipitate was further washed with toluene and dried under reduced pressure to give 13.4 g of the title compound (yield: 60%). APCI-TOF-MS (m/z)=403

Synthesis of 2,6-bis(4-(bis(3,5-difluorophenyl)phosphinoyl)phenyl)-4-(4-phenylphenyl)pyridine

2,6-Bis(4-bromophenyl)-4-(4-phenylphenyl)pyridine (498 mg, 0.92 mmol), F4DPPO (603 mg, 2.2 mmol), palladium acetate (20.7 mg, 0.092 mmol), dppp (75.9 mg, 0.184 mmol), and diisopropylethylamine (2.9 mL) were added to DMSO (5.8 mL) and stirred at 100° C. for 22 hours. After the completion of the reaction, the mixture was poured into a saturated ammonium chloride aqueous solution and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography to give 758 mg of a pale green solid. The resulting solid was recrystallized from heptane/ethanol to give 341 mg of the title compound (yield: 40%). APCI-TOF-MS (m/z)=928

Synthesis of 2,2′,7,7′-tetrakis(bis(3,5-difluorophenyl)phosphinoyl)-9,9′-spirobifluorene

2,2′,7,7′-Tetrabromo-9,9′-spirobifluorene (134 mg, 0.21 mmol), F4DPPO (348 mg, 1.27 mmol), palladium acetate (20.7 mg, 0.092 mmol), dppp (75.9 mg, 0.184 mmol), and diisopropylethylamine (1.34 mL) were dissolved in DMSO (2.8 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into a saturated ammonium chloride aqueous solution and extracted with dichloromethane. The organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The resulting residue was purified by column chromatography to give 132 mg of a white solid. The resulting crystals were further recrystallized from heptane/ethanol to give 102 mg of the title compound (yield: 35%). APCI-TOF-MS (m/z)=1405

Synthesis of 1,3-bis(bis(3,5-difluorophenyl)phosphinoyl)-5-(carbazole-9-yl)benzene

1,3-Dibromo-5-(carbazol-9-yl)-benzene (800 mg, 2.0 mmol), bis(3,5-difluorophenyl)phosphine oxide (1.32 g, 4.8 mmol), palladium acetate (45 mg, 0.2 mmol), dppp (170 mg, 0.4 mmol), and DIEA (0.52 g, 4 mmol) were added to DMSO (8.0 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The obtained organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The resulting residue was purified by column chromatography, and the resulting solid was recrystallized from toluene/methanol to give 950 mg of the title compound (yield: 60%). APCI-TOF-MS (m/z)=788

Synthesis of 1-2,2,4-bis(4-(bis(4-cyanophenyl)phosphinoyl)phenyl)-6-phenyl-1,3,5-triazine

2,4-Bis(4-bromophenyl)-6-phenyl-1,3,5-triazine (561 mg, 1.2 mmol), 2pCNPh-PHO (908 mg, 3.6 mmol), palladium acetate (8.1 mg, 0.036 mmol), dppp (29.7 mg, 0.072 mmol), and DIEA (1.3 mL) were added to DMSO (4.8 mL) and stirred overnight at 100° C. The reaction mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography. The resulting solid was recrystallized from toluene/methanol to give 311 mg of the title compound (yield: 32%).

Synthesis of 1,4-bis(3,5-dicyanophenyl(phenyl)phosphinoyl)benzene

1,4-Bis(phenylphosphinoyl)benzene (219 mg, 0.672 mmol), 5-bromo-1,3-benzene dicarbonitrile (418 mg, 2.02 mmol), palladium acetate (15.1 mg, 0.0672 mmol), dppp (55.3 mg, 0.134 mmol), and DIEA (2.2 mL) were added to DMSO (4.4 mL) and stirred overnight at 100° C. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography. The resulting solid was recrystallized from cyclohexane/ethanol to give 271 mg of the title compound (yield: 31%). APCI-TOF-MS(m/z)=579

Synthesis of 2,7-bis(3,5-dicyanophenyl(phenyl)phosphinoyl)-9,9-dimethylfluorene

2,7-Bis(phenylphosphinoyl)-9,9-dimethylfluorene (371 mg, 0.838 mmol), 5-bromo-1,3-benzene dicarbonitrile (520 mg, 2.51 mmol), palladium acetate (28.1 mg, 0.125 mmol), dppp (103 mg, 0.251 mmol), and DIEA (4.2 mL) were added to DMSO (8.4 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography, and the resulting solid was purified by sublimation to give 240 mg of the title compound (yield: 41%). APCI-TOF-MS (m/z)=694

Synthesis of 3,5-difluorophenyl-diphenanthrylphosphine oxide

3,5-Difluorophenyl-1-bromobenzene (579 mg, 3.0 mmol), diphenanthrylphosphine oxide (1.01 mg, 2.5 mmol), palladium acetate (16.8 mg, 0.075 mmol), dppp (61.9 mg, 0.15 mmol), and DIEA (1.7 mL) were added to DMSO (3.3 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography, and the resulting solid was recrystallized from toluene to give 411 mg of the title compound (yield: 32%). APCI-TOF-MS (m/z)=515

Synthesis of 3,5-di(trifluoromethyl)phenyl-diphenanthrylphosphine oxide

3,5-Difluorophenyl-1-bromobenzene (879 mg, 3.0 mmol), diphenanthrylphosphine oxide (1.01 g, 2.5 mmol), palladium acetate (16.8 mg, 0.075 mmol), dppp (61.9 mg, 0.15 mmol), and DIEA (1.7 mL) were added to DMSO (3.3 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography, and the resulting solid was recrystallized from toluene to give 430 mg of the title compound (yield: 28%). APCI-TOF-MS (m/z)=615

Synthesis of 4-cyanophenyl-diphenanthrylphosphine oxide

3,5-Difluorophenyl-1-bromobenzene (546 mg, 2.5 mmol), diphenanthrylphosphine oxide (1.01 g, 3.0 mmol), palladium acetate (16.8 mg, 0.075 mmol), dppp (61.9 mg, 0.15 mmol), and DIEA (1.7 mL) were added to DMSO (3.3 mL) and stirred at 100° C. for 24 hours. After the completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was dried over magnesium sulfate, and then, concentrated under reduced pressure. The resulting residue was purified by column chromatography, and the resulting solid was recrystallized from toluene to give 504 mg of the title compound (yield: 40%). APCI-TOF-MS (m/z)=504

Example 3: Production and Evaluation of the Layered-Type Organic EL Element (Organic Electroluminescent Element)

1. Production of the Layered-Type Organic EL Element (Organic Electroluminescent Element)

A layered-type organic EL element (red phosphorescent element or green fluorescent element) with an area of 0.023 cm² was formed on a 150 nm ITO glass (50 mm square, GEOMATEC Co., Ltd.). The substrate was treated by ultrasonic cleaning with an alkali detergent (manufactured by Kanto Chemical Co., Inc.), ultrapure water, and acetone (manufactured by Wako Pure Chemical Industries, Ltd.) (each 5 minutes), washed by boiling (5 minutes) in 2-propanol (manufactured by Wako Pure Chemical Industries, Ltd.), and then, washed by UV/O₃ (15 minutes).

(1) Red Phosphorescent Element

The organic film of the hole injection layer was deposited by a spin coating method and the organic films of the hole transport layer, the light emitting layer, the hole blocking layer, and the electron transport layer were deposited by a vacuum vapor deposition method on ITO glass serving as a positive electrode. The material and film forming conditions of each layer are as follows. The ratio of a host material and a guest material of the doped light emitting layer was adjusted by monitoring using a quartz oscillator film thickness meter and adjusting the vapor deposition rate.

-   -   Hole injection layer: PEDOT-PSS (AI4083 (manufactured by         Heraeus)) (40 nm)

Spin coating: 2700 rpm×45 sec (air atmosphere)

Baking: 200° C., 60 min (air atmosphere)

-   -   Hole transport layer: α-NPD         (N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine: see the         following) (40 nm)

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10⁻⁴ Pa

-   -   Light emitting layer (30 nm)

Host material: Organic electron transport material synthesized in Example 2

Luminescent material (guest material): Ir(piq)₃ (see the following), 6% by weight

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10⁻⁴ Pa

-   -   Hole blocking layer: BCP         (2,9-dimethyl-4,7-diphenylphenanthroline: see the following) (10         nm)

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10⁻⁴ Pa

-   -   Electron transport layer: Alq₃ (tris(8-quinolinolato)aluminum:         see the following) (30 nm)

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10 Pa

(2) Green Fluorescent Element

The organic films of the hole transport layer, the light emitting layer, and the electron transport layer were deposited by a vacuum vapor deposition method on ITO glass serving as a positive electrode. The material and film forming conditions of each layer are as follows. The ratio of a host material and a guest material of the doped electron transport layer was adjusted by monitoring using a quartz oscillator film thickness meter and adjusting the vapor deposition rate.

-   -   Hole transport layer: α-NPD (40 nm)

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10⁻⁴ Pa

-   -   Light emitting layer: Alq₃ (20 nm)

Vapor deposition rate: 1 Å/sec, chamber pressure: <1×10⁻⁴ Pa

-   -   Electron transport layer: Organic electron transport material         (30 nm) synthesized in Example 2 or synthesized organic electron         transport material doped with Liq at 10% by weight

A high vacuum vapor deposition apparatus with a chamber pressure of <1×10⁴ Pa was used for vapor deposition of the negative electrode (LiF/Al). The vapor deposition rate was set to 0.1 Å/s for LiF and 5 Å/s for Al. Sealing was carried out in an N₂ atmosphere for all elements. After the completion of deposition of the negative electrode, each element was immediately transferred to a glove box (manufactured by vac; moisture concentration: 1 ppm or less; oxygen concentration: 1 ppm or less) purged with nitrogen, and the element was sealed with a glass cap (manufactured by Climbing Co., Ltd.) to which a dry sheet material was attached (Dynic Corporation).

2. Measurement of the Lifetime of the Layered-Type Organic EL Element

The layered-type organic EL element was placed in a constant thermostatic bath at 25° C. for measurement of changes in brightness and voltage associated with constant current continuous driving by a lifetime measuring apparatus (manufactured by Kyushu Keisokki Co., Ltd.).

3. Results

Tables 2 and 3 list the measurement results. Table 2 lists the evaluation results of the organic electron transport materials (I) to (XI) for the red phosphorescent element. Table 3 lists the evaluation results of the organic electron transport materials (I) to (XI) for the green fluorescent element. Moreover, the results of the Comparative Examples using the DPPO2DMFlu as an organic electron transport material are also listed. The description in parentheses in the column of half-life measurement indicates short circuit during the measurement in each of Tables 2 and 3. As is understood from the tables below, the organic electron transport materials (I) to (XI) have long lifetime and excellent durability.

TABLE 2 Half lifetime Material (relative value) I 236 II 53 III 154 IV 586 V 19 VI 2.3 VII 18 X 57 XI 168 Comparative Example 4.3

TABLE 3 Half lifetime Material (relative value) I 115 I: Liq 558 II 163 II: Liq 105 III 53 III: Liq 196 Comparative Example 1

The foregoing describes some example embodiments for explanatory purposes. Although the foregoing discussion has presented specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. This detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is defined only by the included claims, along with the full range of equivalents to which such claims are entitled.

This application claims the benefit of Japanese Patent Application No. 2015-257189, filed on Dec. 28, 2015, the entire disclosure of which is incorporated by reference herein.

REFERENCE SIGNS LIST

-   1 Organic EL element -   2 Transparent substrate -   3 Positive electrode -   4 Hole transport layer -   Light emitting layer -   6 Electron transport layer -   7 Negative electrode -   8 Sealing member 

1. An organic electron transport material, which comprises a phosphine oxide derivative represented by the following Formula (1):

where, in Formula (1), R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms:

where, in Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.
 2. The organic electron transport material according to claim 1, wherein the electron withdrawing group is selected from the group consisting of a halogen atom, a cyano group, a nitro group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, and a trifluoromethyl group.
 3. An organic electroluminescent element, which comprises: a pair of electrodes composed of a positive electrode and a negative electrode; and a light emitting layer that is formed between the pair of electrodes so as to allow holes and electrons to be injected from the electrodes directly or via a different thin film layer and is electrically excited by recombination of holes and electrons injected from the electrodes so as to emit light, wherein the organic electroluminescent element contains a phosphine oxide derivative represented by the following Formula (1) and includes an electron transport layer that is disposed to be in contact with the negative electrode side of the light emitting layer:

where, in Formula (1), R¹ represents an atomic group, which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms:

where, in Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.
 4. The organic electroluminescent element according to claim 3, wherein the electron withdrawing group is selected from the group consisting of a halogen atom, a cyano group, a nitro group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, and a trifluoromethyl group.
 5. An organic electroluminescent element, which comprises: a pair of electrodes composed of a positive electrode and a negative electrode; a compound that is formed between the pair of electrodes so as to allow holes and electrons to be injected from the electrodes directly or via a different thin film layer and is electrically excited by recombination of holes and electrons injected from the electrodes so as to emit light; and a light emitting layer that contains a phosphine oxide derivative represented by the following Formula (1):

where, in Formula (1), R¹ represents an atomic group which has one or more of either or both of aryl and heteroaryl groups, one of the one or more of aryl and heteroaryl groups being directly bound to a phosphorus atom, and which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, Ar¹ and Ar² each independently represent any of an aryl group, a heteroaryl group, a substituted aryl group, and a substituted heteroaryl group which may have a phosphine oxide group represented by the following Formula (2) on any one or more of carbon atoms, and at least one of R¹, Ar¹, and Ar² has an electron withdrawing group having a positive Hammett σ value on any one or more of carbon atoms:

where, in Formula (2), R² to R¹¹ each independently represent a hydrogen atom or the electron withdrawing group.
 6. The organic electroluminescent element according to claim 5, wherein the electron withdrawing group is selected from the group consisting of a halogen atom, a cyano group, a nitro group, a carboxyl group, a formyl group, a carbonyl group, an alkoxycarbonyl group, and a trifluoromethyl group. 